Method for coating liquid compositions employing electrostatic field



w. F. DE GEEST ETAL 3,335,026 METHOD FOR COATING LIQUID COMPOSITIONS EMPLOYING ELECTROSTATIC FIELD Filed July 16, 1965 Aug. 8, 1967 INVENTOR ATTORNEY United States Patent 3,335,026 METHOD FOR COATING LIQUID COMPOSITIONS EMPLOYING ELECTROSTATIC FIELD Wilfried Floreut De Geest, Berchem-Antwcrp, and Paul August Verkinderen, Edegem, Belgium, assignors to Gevaert Photo-Producteu N.V., Mortsel-Antwerp, Belgium, a Belgian company Filed July 16, 1963, Ser. No. 295,550

6 Claims. (Cl. 11793.4)

This invention relates to a method of coating liquid compositions and, more particularly, to an improved method of coating by means of a meniscus coater.

Many apparatus for coating webs are known.

One of the most simple amongst them, which is used on an extensive scale in the photographic industry for the manufacture of photographic film and paper, is the socalled meniscus or bead coater. Said coater consists of a pan containing a supply of photographic emulsion, a coating roller which is provided above the pan and about which the web is passed, and some idler rollers for leading the web to and from the coating roller. A clearance of about mils exists between the support and the emulsion surface. When coating is started, the bead is caused to adhere to the web by briefly reducing said clearance to zero by agitating the liquid supply or by raising its level. Once the bead is formed, the coating weight is controlled by the speed of the web and/ or by metering devices such as a trailing knife, air knife, etc. The surface tension of the emulsion must be kept constant since changes will affect coat weight and surface quality.

Although the just mentioned apparatus is simple of construction, its use involves some serious problems.

A first difficulty is to keep the level of the emulsion strictly constant, since variations of the said level may lead to a partial or total disruption of the coating bead. For keeping the level of the emulsion constant, the known overflow systems do not always suflice, and complicated control systems have to be provided, comprising level measuring means, servo amplifiers, volumetric pumps, etc.

Another disadvantageous consequence of variations in the level of the emulsion if the level of the emulsion rises high enough, the emulsion will be applied to the back side of the web.

Similarly application of emulsion to the back side of the web may be caused by the tendency for the coating to squirt around the edges of the web on to the back side of the web. This phenomenon may be eliminated by preventing the margins of the web from being coated. This has been done in the prior art by means of edge doctors or dams for controlling uncoated margins of the sheet but these means all require accurate adjustment which are the more diflicult to make by reason of the rather inaccessible locations such means must occupy.

The same difliculties with the edge doctors or dams arise, of course, when webs of different widths must be coated.

- The object of the invention is to provide a coater which is an improvement over, and overcomes the noted disadvantages in the conventional meniscus coater.

According to the present invention, the method of coating a liquid composition onto a web by means of a meniscus coater comprises the steps of increasing the clearance between the Web and the liquid composition so that the surface tension of the liquid composition is no longer capable of maintaining the meniscus between the liquid composition and the web, and of applying a potential difference between the liquid composition and the coating roller so as to establish the meniscus between the web and the emulsion.

The invention will now be described in terms of nonlimitative illustrative embodiments of the accompanying drawings, of which:

Patented Aug. 8, 1967 "ice FIG. 1 is a diagrammatic vertical sectional view of an the line 1-1 of FIG. 2.

FIG. 2 is a diagrammatic horizontal sectional view of the apparatus, taken on the line 22 of FIG. 1, the air knife being omitted.

FIG. 3 is an enlarged-sectional view representing the meniscus.

FIG. 4 is a diagrammatic view of a coating apparatus, the coating roller of which is provided with an insulating layer.

FIG. 5 is a diagrammatic view of a two-roller meniscus coater.

FIGS. 1 and 2 represent two diagrammatic sectional views of a head or meniscus coater which is extensively used in the coating of photographic emulsions.

The coater comprises the coating roller 13, the idler rollers 12, 19 and 20, the coating pan 17 containing the light-sensitive photographic emulsion 16, and the air knife 18.

The emulsion 16 is introduced into the pan at the inlet opening37 and carried off via the overflow 36, through the outlet opening 38. In contrast to the common meniscus coaters, the clearance between the coating .roller 13 and the emulsion 16 is increased to such a value, that no meniscus can be formed, for the given emulsion, between the emulsion and the support 11 which is passed about the coating roller.

According to the invention, the meniscus between the support 11 and the emulsion 16 is established by applying a voltage difference between the coating roller 13 and the emulsion supply 16.

To this end, the metal coating roller is mounted in an insulated way by means of the insulating bearings 14 and 15.

Means is provided (not shown in the figures) for making electrical contact with the rotating shaft of the coating roller, such means being connected to one terminal of a source of electrical potential 21. The other terminal of said source, as well as the coating pan 17 and all the other parts of the apparatus, are grounded.

The potential difference between the coating roller and the emulsion supply creates an electrostatic field which traverses the support 11 and the air gap between the support and the emulsion supply. The said electrostatic field establishes the liquid meniscus between the support and the emulsion, since a liquid bead of the latter is literally struck or sucked up against the support, as represented by the drawn lines 23, 24 in FIG. 3, when the electric field is applied.

The order of magnitude of the clearance depends in the first place upon the magnitude of the voltage difference which is applied. In the present case, the clearance amounted to 2 mm., and the applied potential difference was a DC. tension of 1000 volts.

The electrostatic field traversing the support has also a favourable action on the neutralization of the occasional charge distributions static electricity in and on the surface of the support being coated, and in practice less difficulties are encountered with the present way of coating than with the known meniscus coater.

While operating the apparatus according to the method of the invention, it has been found that the level of the emulsion may undergo slight variations without disturbing the coating process. Even slight agitation of the emulsion supply, e.g. caused by vibrations, does not have any effect on the quality of coating.

The insensitivity to disturbances of the level and for stability of the emulsion supply, are the most remarkable improvements of the coating method according to the invention.

Complicated systems for controlling the level of the emulsion in the coating pan may be omitted, and minute adjustments for arranging the coating roller on an absolutely horizontal axis so as to run parallel to the emulsion surface are no longer required.

The potential source 21 may be any electric or electronic power supply, capable of delivering voltages with frequencies ranging from DC. to cycles, and with values situated between 500 and 10,000 volts. Said values are in no way limitative. Higher voltages may e.g. be applied, taking into account, however, that arcing or ionization between the coating roller and the emulsion supply cannot be tolerated.

The voltage of the source 21 may undergo fluctuations without influencing the coating process. However, the said fluctuations should not exceed a given value, since otherwise the thickness of the coated layers may undergo variations. This is especially the case in those instances Where no after-metering of the coating is provided. In such instances the voltage may even control the thickness of the coated layers.

The current which has to be delivered by said sources is but very small, since no true closed circuit exists. Leakage currents which exist at the insulating areas and currents which arise owing to disturbances in the electrostatic field caused by static charges in the supports, on the surface thereof, and by changes in the dielectric properties of the supports, result in a total current which in practice does not exceed 10 ma.

The potential source 21 may consist of a simple separation transformer connected to the supply main, followed by a metal rectifier and a smoothing filter; an electronically stabilized power supply capable of delivering voltages which can continuously or step-wise be changed within an extended range; or a LP. or HP. generator, etc.

It has to be noticed that in case a DC. potential differonce is applied, the coating roller may be positive as well as negative in respect of the liquid supply to be coated. In some cases, however, a given polarity yields slightly better results than the opposite polarity, and the right choice in such cases has to be based on empirical tests.

It appears from FIGURE 2, that the length of the coating roller 13 is slightly smaller than the width of the support 11, so as to let the support extend with its margins outside the limits of said roller. The width of the extending margin portions of the support amounts to a few mil limetres.

The consequence of the arrangement just described is that no electrostatic field traverses the said margin portions and consequently no emulsion is applied onto those places.

The coated part of the support is indicated in FIG. 2 by the part 26 hatched in broken lines.

The exposed margin portions perform the function of a screening member, preventing the emulsion from making contact with the side walls of the coating roller and establishing a conductive path between the roller and the emulsion which would lead to a drop in the applied potential diflFerence and consequently to a rupture or a disturbance of the meniscus. In case the meniscus would not be ruptured or disturbed, the electric current would in any case disadvantageously influence the coated photographic layer by causing fog formation.

The described embodiment is designed for the coating of liquid compositions on electrically non-conductive supports such as paper, polystyrene, polyethylene terephthalate, and any other web material with a surface resistivity greater than 10 ohm/square.

In case e.g. a very thin paper support is coated with a substance of very low viscosity, it may happen that the said substance penetrates locally through the support and gives rise to an electrically conductive path between the coating roller and the meniscus with the same consequences as mentioned hereinbefore. Furthermore, the support thus rendered conductive causes a dispersal of the electrostatic field which also may contribute to the rupture or disturbance of the meniscus.

An electric current flow and a dispersal of the electrostatic field may also arise, if supports are to be coated have a surface resistivity less than 10 ohm/square (hereinafter called noninsulating supports). Such noninsulating supports are e.g. negative and positive multicolor photographic films comprising a support which on its back side is provided with an antihalation layer rendered conductive by the incorporation of graphite so as to avoid electrostatic charge distributions, supports provided with a conductive anti-stress layer, etc.

Any conductive path between the coating roller and the meniscus, as well as any dispersal of the electrostatic field, in the coating of an electrically non-insulating support is avoided, by providing the metal coating roller with a thin insulating layer and by using an AC. potential diiference between the emulsion supply and the coating roller.

The insulating layer may consist of any suitable material, e.g. polyamides such as Akulon (trademark for a polyamide, marketed by AKU N.V., Arnhem, Holland), polymers such as P.V.C., Teflon (trademark for polytetrafiuorethylene, marketed by E. I. du Pont de Nemours and Co., Wilmington, Del.), Plexiglas (trademark for polymethyl methacrylate, marketed by Rohm and Haas, Philadelphia, Pa., U.S.A.) etc. The thickness of the said layer is in the order of magnitude of some tenths of a millimetre.

Where the support is conductive, the use of an AC. is imperative, since the electrical equivalent of the device is the parallel connection of a resistor with a capacitor, which is connected to the voltage source (the capacitor representing the capacitance between the coating roller and the emulsion supply, the resistor representing the resistance of the electrically conducting support in longitudinal direction).

The better the conductivity of the support, the higher the frequency of the source should be for concentrating most of the electrical energy in the capacitive member of the equivalent parallel connection, and for consequently yielding a highly efficient process.

An apparatus which embodies the features described above is represented in FIG. 4. The metal coating roller 13 has a diameter of 100 mm., and is provided with an insulating P.V.C. layer with a thickness of 1 mm. The said layer has been provided on the coating roller by passing a sleeve of soft P.V.C. over the roller, and by next putting the roller in an oven with a temperature of C., until the sleeve had tightly shrunk around the periphery of the roller. The roller 13 is mounted in an insulated way and is connected to the potential source 21. The thickness of the coated layer is in the present embodiment controlled by the stainless steel pressure roller 28 mounted at a fixed distance from the periphery of the coating roller 13.

The voltage source 21 is connected between the coating roller and the pan 17 containing the emulsion supply 16. The pan as well as the metal roller 28 are grounded.

In the present case a L.F. tension of 8000 v.-6 kc. was applied. The clearance amounted to 5 mm.

A third embodiment of the invention is shown in FIG. 5. This figure represents a meniscus coater of the tworoller type. It comprises the coating roller 13, the pan 17 containing the liquid emulsion 16, and the roller 29. The roller 29 is covered with a liquid bead 32. The meniscus 33 between said liquid bead and the support 11 is formed by the potential difference applied between the insulated coating roller 13 and the other parts of the apparatus, i.e. the pan 17, roller 29, etc., which are grounded.

The appratus is not followed by a metering or smoothing device, so that coat weight is controlled almost exclusively by speed and by applied tension.

Of course, the problem of the liquid level does not exist in the present apparatus, since the roller 29 dips with a considerable part of its periphery in the emulsion supply. The difliculty of the adjustment of the clearance between rollers 13 and 29, however, is in this apparatus completely eliminated.

In case a conductive support as defined hereinbefore should be coated for the reasons already mentioned, the coating roller 13 may be covered with an insulating layer.

It is clear that the described apparatus as such differs only slightly from the known meniscus coaters, and that simple measures suflice for rendering existing coaters suitable for the application of the coating method according to the invention.

The said measures include the insulated mounting of the bearings of the coating roller, the provision of a contact strip for contacting the said insulated roller, the provision of a voltage source, the increasing of the dis tance between the web and the emulsion, etc.

The coating pan as well as all other parts of the apparatus are grounded, so that the protective screening of the coating roller can be realized in a most simple way.

The examples given hereinbefore related to the coating of photographic emulsions. It is clear that the invention is not limited to a method of coating merely light-sensitive photographic emulsions, but includes the coating of all other liquid compositions having a specific conductivity of the magnitude of at least per cc.

The invention can also be practised for the coating of antistress layers, antihalation layers, color filter layers etc. in the production of photographic material, for the coating of zinc oxide and similar photocon-ductive layers in the production of electrophotographic recording material, for the coating of thermosensitive layers in the production of thermophotographic recording material, for the coating of image-receiving layers in the production of an image-receiving material suited for use in the silver halide diffusion transfer process, for the coating of electrically conductive or uoninsulating layers on supports with very good insulation properties in the production of electrostatic recording material, for the coating of stripping layers in the production of photographic stripping films for use in photomechanic reproduction processes, for the coating of.magnetizable layers in the production of magnetic recording tape, for the coating of subbing layers, for the production of X-ray film, for the coating of plastics, paints, resins on flexible supports, which themselves may ormay not be plastics, etc.

What we claim is:

1. The method of coating a liquid composition on a moving web which comprises the steps of:

(1) maintaining a supply of said liquid composition with at least a portion of its surface exposed, the

width of said portion being at least substantially equal to the portion of the width of the web to be coated,

(2) guiding said web along a predetermined path passing over a coating roller at a locus in close proximity to the exposed surface portion of said liquid supply and on the opposite side of said web from said liquid supply, the clearance between said web at said locus and said exposed surface being in excess of the distance the surface tension of said liquid composition is normally capable of maintaining a meniscus, and

(3) maintaining an electrical potential difference between the liquid supply and said coating roller at said locus of such magnitude as to establish a meniscus between the web and the liquid surface.

2. The method of claim 1 wherein said web while passing through said locus is isolated from electrical contact with said coating roller.

3. The method of claim 1 wherein the width of the exposed surface portion of said liquid is less than the width of the web so that the margins of the web extend outside the confines of the liquid supply surface, preventing the formation of a meniscus at such margins.

4. The method of coating according to claim 1, wherein the said potential difierence is an alternating current potential difference.

5. The method of coating according to claim 1, wherein the said potential difference is a direct current potential difierence.

6. The method of coating according to claim 1, comprising controlling the coat weight by varying the magnitude of the applied voltage difference.

References Cited UNITED STATES PATENTS 2,052,131 8/19 36 Chappell 117-93.4 X 2,254,016 8/1941 Melton et al. 118-627 2,809,128 10/1957 Miller 117-93.42 2,877,740 3/1959 Friderici 118-635 X 2,952,559 9/1960 Nadeau 118-638 3,206,323 9/1965 Miller et al. 117--93.4

ALFRED L. LEAVITT, Primary Examiner.

A. GOLIAN, Assistant Examiner. 

1. THE METHOD OF COATING A LIQUID COMPOSITION ON A MOVING WEB WHICH COMPRISES THE STEPS OF: (1) MAINTAINING A SUPPLY OF SAID LIQUID COMPOSITION WITH AT LEAST A PORTION OF ITS SURFACE EXPOSED, THE WIDTH OF SAID PORTION BEING AT LEAST SUBSTANTIALLY EQUAL TO THE PORTION OF THE WIDTH OF THE WEB TO BE COATED, (2) GUIDING SAID WEB ALONG A PREDETERMINED PATH PASSING OVER A COATING ROLLER AT A LOCUS IN CLOSE PROXIMITY TO THE EXPOSED SURFACE PORTION OF SAID LIQUID SUPPLY AND ON THE OPPOSITE SIDE OF SAID WEB FROM SAID LIQUID SUPPLY, THE CLEARANCE BETWEEN SAID WEB AT SAID LOCUS AND SAID EXPOSED SURFACE BEING IN EXCESS OF THE DISTANCE THE SURFACE TENSION OF SAID LIQUID COMPOSITIONM IS NORMALLY CAPABLE OF MAINTAINING A MENISCUS, AND (3) MAINTAINING AN ELECTRICAL POTENTIAL DIFFERENCE BETWEEN THE LIQUID SUPPLY AND SAID COATING ROLLER AT SAID LOCUS OF SUCH MAGNITUDE AS TO ESTABLISH A MENISCUS BETWEEN THE WEB AND THE LIQUID SURFACE. 